1. Field of the Invention
The present invention relates to a tube type freezing unit of the type in which sherbet-like slush is made from a low-temperature heat accumulating solution filling a plurality of thin tubes disposed within a shell, and an intube freezing method therefor.
2. Description of the Related Art
An icy low-temperature heat accumulating system is more advantageous than a liquid low-temperature heat accumulating system in that it can be made compact due to its small heat-accumulating capacity. Since bulky ice is not easy to convey, the icy low-temperature heat accumulating system makes and employs slush, which has a high fluidity and which can be conveyed easily.
Conventional icy low-temperature heat accumulating systems of the type in which slush made within tubes is conveyed to and stored in a low-temperature heat accumulating tank include the thin-film dropping type freezing system, the circulatory freezing system, the freezing system which utilizes the supercooling effect of water, and so on.
FIG. 1 shows the conventional freezing system of the thin-film dropping type, and FIG. 2 shows the freezing system which utilizes the supercooling effect of water. In FIGS. 1 and 2, a reference numeral 61 denotes a freezer, 62: a freezer head, 63: a circulating solution receiver, 64: a circulating solution inlet, 65: a circulating solution outlet, 66: a refrigerant outlet, 67: a refrigerant inlet, 68: tubes, 71: a freezer, 72: a brine cooler, 73: a supercooling device, 74: a filter, 75: a low-temperature heat accumulating tank, and 76: a secondary system.
The thin-film dropping type freezing system has been developed by CBI. It employs a longitudinal shell-&-tube type heat exchanger as a freezer, as shown in FIG. 1. The shell is constituted by the flooded, direct expansion type freezer 61 with the tubes 68 incorporated therein. The freezer 61 employs a refrigerant. The refrigerant is introduced into the shell from the refrigerant inlet 67, and is drawn out from the refrigerant outlet 66 in a gaseous form. The inner surface of each of the tubes is specular. An aqueous solution of ethylene glycol which serves as a low-temperature heat accumulating solution is fed to the freezer head 62 from an icy low-temperature heat accumulating tank through the circulating solution inlet 64. When the aqueous solution of ethylene glycol reaches the level of the tubes 68, it flows into the tubes and then drops along the specular inner surfaces of the tubes 68, during which time heat is exchanged between the aqueous solution of ethylene glycol and the refrigerant to cool the aqueous solution of ethylene glycol. This causes the water molecules in the brine to freeze, generating fine icy crystals. The sherbet-like slush thus formed drops to the circulating solution receiver 63 located at the lower portion of the freezer 61. The slush received by the circulating solution receiver 63 is then conveyed to and stored in the icy low-temperature heat accumulating tank.
The circulatory freezing system has been developed by a Canadian company, Sunwell Engineering. In this freezing system, an aqueous solution of ethylene glycol which serves as a low-temperature heat accumulating solution is circulated within the freezing tubes in a jacket which is a direct expansion type evaporator. In this way, a high pressure is generated on the freezing surface located on the outer periphery, and slush is formed at a low-pressure central portion due to the supercooling effect.
The freezing system which utilizes the supercooling effect of water employs the filter 74, as shown in FIG. 2, for the purpose of cleaning the low-temperature heat accumulating solution, which is necessary to maintain unstable supercooling effect of water. It adopts an indirect cooling method by means of a brine, which is required to provide a stable cooling temperature, pressurization within the tubes, and conveyance of the solution at a high speed.
Thus, in the conventional freezing systems, the sherbet-like slush is made by the refrigerator during the low-temperature heat accumulation operation utilizing the characteristics of the aqueous solution of ethylene glycol, and the slush made is stored in the low-temperature heat accumulating tank, the low-temperature heat thus accumulated being released as a low-temperature heat source through a heat exchanger when necessary.
However, the above-described conventional freezing systems have disadvantages in that they have to use the aqueous solution of ethylene glycol which is an industrial waste to achieve a low freezing point and thereby solve the problem involving the attachment of ice on the freezing surface and in that they require large auxiliary power. The freezing system of the type which utilizes the supercooling effect of water requires a large amount of energy and high cost to achieve indirect cooling and to convey the low-temperature heat accumulating solution, which are required to maintain the supercooling effect. These reduce the efficiency of the overall system.
An aqueous solution of ethylene glycol of a low concentration of 10% or less becomes moldy at 30.degree. C. or less. This makes use of the aqueous solution of ethylene glycol as heat-accumulating material for a heating system difficult. Hence, if it is used in a heat pump, heat accumulation has to be suspended in winter, or the aqueous solution of ethylene glycol has to be substituted with plain water. Alternatively, a high-temperature heat accumulating tank has to be provided separately from a low-temperature heat accumulating tank for a cooling system. Furthermore, the aqueous solution of ethylene glycol is designated as an industrial waste, and is therefore generally used indirectly through a heat exchanger. This makes use of it very difficult as a circulating solution for an open type heating tower because of the possibility of the circulating solution scattering and leaking.
In a heating tower system, the water vapor in the air enters the circulating solution to reduce the concentration thereof. Hence, the concentration of a non-freezing liquid in the circulating solution must be increased by operating a heating tower and a pump on days of good weather and low humidity while stopping the operation of the heat pump and thereby thickening the circulating solution by natural evaporation. The circulating solution may also be thickened by heating part of it and thereby causing the water therein to evaporate. Generally, thickening is achieved by heating rather than by the natural evaporation, and this makes the overall system complicated and less efficient.